{omsagent} role assignment changes for msi (#1)

[rashmichandrashekar]

Role assignment changes to assign role for msi if it is created for omsagent addon

[azuresdkci]

If this PR is for a new extension or change to an existing extension, use the following to try out the changes in this PR:

docker run -it microsoft/azure-cli:latest
export EXT=<NAME>
pip install --upgrade --target ~/.azure/cliextensions/$EXT "git+https://github.com/rashmichandrashekar/azure-cli-extensions.git@master#subdirectory=src/$EXT&egg=$EXT"

[yonzhan]

add to S165.

[mmyyrroonn]

Hello. Like we talked offline, could you change the help message in extension too? Better to have same code with the main azure cli

[rashmichandrashekar]

Hello. Like we talked offline, could you change the help message in extension too? Better to have same code with the main azure cli

I have added the changes in the help too

[mmyyrroonn]

Same with this one. Azure/azure-cli#12100 (review) LGTM. Could you please update the index.json in the same PR?